Susan belongs to a group of Doctor Who companions not from present-day Earth. Like the Doctor, Susan once called Gallifrey her home, and while the Timeless Child storyline perhaps altered the Doctor's ...

The 1:1 ratio results from the way the X and Y chromosomes are doled out in sperm and eggs. We know that boys and girls are produced in much the same frequency. But how – and why – is this 1:1 ...